This invention relates to an improved process for preparing lignosulfonate additives for drilling fluids used in the drilling of oil and gas wells. The invention also relates to drilling mud additives which comprise a mixture of zirconium and titanium lignosulfonates which are substantially free of chromium.
U.S. Pat. No. 4,220,585 to Paul H. Javora and Bethel Q. Green, incorporated herein by reference, describes chromium-free drilling fluid additives effective as viscosity controlling agents. The additives are composed of complex lignosulfonates containing titanium and/or zirconium. The additives are stated to be, in many cases, more effective viscosity controlling agents than the chromium or chromium-iron lignosulfonates which are widely used in the drilling industry. The additives have the additional advantage of avoiding the toxic nature ascribed to chromium lignosulfonates.
Lignosulfonates, in accordance with U.S. Pat. No. 4,220,585, are prepared by reacting lignin liquors obtained from pulping wood with salts of the desired metal or metals. When necessary, the precipitated material can be removed. Oxidation of the material, which modifies certain properties such as the thinning or reduction of the viscosity of clay suspensions and reduction of the gel-like properties of such suspensions, can be any one of the process steps.
In the commercial preparation of the drilling fluid additives of U.S. Pat. No. 4,220,585, normally, the lignosulfonate is purchased as the calcium salt. Most of the calcium must be removed, because calcium adversely affects the viscosity control properties of the final product. A metal sulfate is added to precipitate the calcium as calcium sulfate and to form metal lignosulfonates. Typically, the calcium sulfate is removed by filtration. The added metal sulfate may be in the form of zirconium sulfate and/or titanium sulfate. Sulfuric acid may also be added to aid in precipitating the calcium sulfate.
Because of the chemically complex nature of the sulfonated lignin material prepared in accordance with U.S. Pat. No. 4,200,585, their exact chemical composition is not readily ascertainable. Consequently, reference to these compositions as "lignosulfonates" does not imply a limitation to salts formed by base-exchange chemical reactions. They may also include chelates as well as other metal complexes.
When making drilling fluid additives according to U.S. Pat. No. 4,220,585, it has been found that the filtration rate drops to zero because of total blinding or plugging of the filter cloth. Further processing of the lignosulfonate solution becomes impractical. Even with continuous cleaning of the filter cloth, only limited production is achieved. Consequently, production of these chromium-free lignosulfonate drilling fluid additives is inefficient and unduly expensive.
U.S. Pat. No. 3,544,460, to Aaron Markham and Kenneth Blackmore, relates to a sulfonated lignin-containing material and its use as an additive in drilling muds.
U.S. Pat. No. 3,634,387, to Walter Dougherty, relates to a sulfomethylated lignin-ferrochrome complex and a process for making it. A ferrochrome salt solution is used to make the lignin-containing complex. The ferrochrome salt solution is made by mixing stoichiometric amounts of a ferrous salt and a dichromate salt in order to give stability to the salt solution. A mineral acid is added to this salt solution to prevent the precipitation of hydroxides of iron or chromium upon mixing the sulfomethylated lignin solution with the salt solution.
U.S. Pat. No. 3,962,099, to Donald Whitfill, relates to a water base drilling mud composition wherein calcium ions are controlled and converted to water-insoluble plant nutrient compounds by the use of monocalcium phosphate compounds. According to this reference, at least a stoichiometric amount of a monocalcium phosphate containing compound must be added to an alkaline earth metal hydroxide.
U.S. Pat. No. 4,447,339, to William Detroit, is directed to a drilling fluid additive comprised of manganese lignosulfonates and a process for making the additive. Also described is the addition of heavy metal cations (such as iron, copper and zirconium). In one version of the process, precipitated calcium sulfate is removed from the mixture prior to complexing the heavy metal cation.
U.S. Pat. No. 4,457,853, also to William Detroit, is a continuation-in-part of U.S. Pat. No. 4,447,339, mentioned above. Boron is added to the manganese lignosulfonate to produce a manganese-boron lignosulfonate.